Various flow meter devices for measuring a flow of a fluid such as a gas by utilizing an ultrasonic wave have been proposed and put into practice. This flow meter device is typically configured in such a manner that a measurement passage is provided with a pair of ultrasonic transmitters/receivers, and ultrasonic pulses are transmitted and received alternately. This makes it possible to measure a flow velocity of a fluid based on a difference between a propagation time in a forward direction and a propagation time in a reverse direction. Therefore, the flow of the fluid can be measured by utilizing the flow velocity and a pipe diameter of the measurement passage.
Such a flow meter device is conventionally aimed at obtaining a highly accurate flow value while achieving low electric power consumption. For example, Patent Literature 1 discloses a flow meter device including a number setting means which changes the number of times alternate transmission/reception between an ultrasonic transmitter and an ultrasonic receiver is repeated, a first time measuring means which counts a signal of a low-frequency oscillator when the repeated transmission/reception starts, a second time measuring means which starts to count a signal from a RF oscillator after a passage of the time set by the first time measuring means and stops counting when the repeated transmission/reception ends, and a flow calculating means which calculates total times using the first time measuring means and the second time measuring means, and derives a flow (flow rate) based on a difference between the total times, and the time set by the first time measuring means is changed based on the value of the number setting means.
In accordance with the above stated configuration, the first time measuring means which is low in frequency, is used for most of a time required for the measurement, while the second time measuring means which is high in frequency is activated only when high accuracy is needed. Therefore, it becomes possible to attain a high resolution in low electric power consumption.    Patent Literature 1: Japanese Patent No. 3689973